The present invention relates to an electrothermal printing unit of an improved kind for impact-less typing of alphanumerical characters, in accordance with point matrices, on to a heat-sensitive recording medium. In this specification "row" refers to a row of points of a row and column matrix in contrast to "line" which refers to a line of characters, a row being parallel to a line.
For efficient utilization of the speed of modern systems of electronic information handling it is essential to have available very high-speed peripheral printing units, and it is also desirable that the printing units should not be too numerous, since they are normally located near the user, in offices and other working places. The number of units required depends on the output of each unit.
The use of printing equipment with ballistic impact of a character-bearing member against an inked ribbon in contact with paper involves speed limitations intrinsic in the mechanical operation and noise levels which often reach the limits of toleration, but the introduction of impactless printing equipment removes these drawbacks.
Impact-less printing devices are known which make use of the electrothermal typing method. For example, U.S. Pat. No. 3,839,630 to E.E. Olander et al, relates to an electrothermal line printer, i.e. designed to type simultaneously a complete line of characters by means of point matrices, on to a heat-sensitive recording medium.
It comprises a printing head which carries, aligned in parallel to the printing line, as many thermally-activable printing elements as there are characters in a line of print, multiplied by the number of points constituting a row of each matrix. For example, if the matrix is of the kind having 7 rows .times. 5 columns and with a line of print having 16 characters, the number of elements required is 16 .times. 5 = 80 elements.
The head, during elementary printing act, carries out the typing of a row of matrix points for all the characters in a line of print. Complete printing of a row of characters therefore requires as many elementary printing acts as there are points in a column of each matrix. During the elementary printing acts, the recording medium is fixed relative to the head, and carries out an interlinear elementary movement equal to the distance between two rows of the matrix after each elementary printing act.
The selection and control circuits must therefore be designed to control a number of printing elements equal to a multiple of the number of characters in a line of print. The printing speed is obviously high and mechanical control is concerned solely with interlinear movement of the recording medium. But the complexity and the number of the electronic control circuits for this kind of printing unit leads to serious electrical connection problems and above all to a very high construction cost which is not always justified by increased performance in terms of printing speed.
These drawbacks are removed by the unit according to the invention, which makes it possible to obtain a speed performance comparable with that of an electrothermal line printer of the kind described above, but using a much smaller number of theremoelements and an elctronic control circuit which is consequently much smaller.